fix: resolve clippy warnings and improve code quality

[strmfos]

Fix Clippy Warnings

• Added #[nexus_rt::main] attribute to main functions
• Fixed custom_input/custom_output attribute formats
• Added conditional compilation for non-riscv32 targets
• Changed function return types to Result
• Extracted complex types into type definitions
• Fixed imports and removed unnecessary references
  All tests passed

[github-actions]

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[strmfos]

I have read the CLA Document and I hereby sign the CLA

[sjudson]

Thanks for the PR. Generally speaking however, we prefer not to merge purely cosmetic changes, so I'm going to close for now.